Picture of How to make a
Now you can buy the kit of this instructable from Jameco!

This project is named "Ghost Knocker". You will get the idea if you watch this movie.

It is an easy prototyping. You can use it on toilets, closets, or office doors.
Make your own, and make someone surprised and laugh!

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Step 1: Check the materials you need.

Picture of Check the materials you need.
Below is the list of stuff you need to make it:

1. Arduino
I used an Arduino Pro Mini 3.3V. You also need USB-Serial tether to program it.

2. A battery box
I used a AA x 4 battery box with power line wires.

3. Breadborad
I used a mini breadboard by Sparkfun.

4. ECM (Electet Microphone) Module
I used an ECM module by Sparkfun.

5. RC Servo motor
I used a mini servo motor EK2-0508 by E-Sky.

6. Jumper wires

7. An Aluminum Electrolytic Capacitor
I used a 100uF capacitor.

8. A knocker hammer parts
I used a small wooden ball toy.

9. A flexible metal wire
It is for connecting the servo horn and the knocker hammer.
A steel wire 2mm in diameter will be suitable.

10. Double-stick tape
It is for making the parts bond, and for mounting the device on the door.

Step 2: Make the knocker hammer.

Picture of Make the knocker hammer.
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Make two holes in to servo horn.

Make the wire to be in hole and bend it to fix on the servo horn.

Bond the ball (or whatever you choose for hammer) to the wire.
Drill the hammer if you need.

Step 3: Mount the breadboard and the servo on the battery box.

Picture of Mount the breadboard and the servo on the battery box.
Mount the breadboard to battery box with double-stick tape.

And make the RC servo fix on the battery box to move the servo horn can move.

mecha20121 year ago
I managed to figure out the coding problem.It turns out something wrong with my computer. I used the computer and no error show. I'm using arduino pro mini. I already build the circuit according to the pic above but its not working. The servo motor give no reaction whenever I knock something. Also when I open the serial monitor it didn't print out anything. But from the coding given, there should be something printed out. I attached pic of the project so far.

Thank you.
longjie0723 (author)  mecha20121 year ago

At first, if you change my code to comment following lines in, you can see analog value from mic by using "serial monitor".

//Serial.print (val);
//Serial.print ("\n");

If you can not see any message on the serial monitor, the arduino doesn't wrok correctly, and I have no idea what happens... Something wrong with the arduino or PC or the connection.

If you can see value between 0 to 100, it means the audio level of the mic.
Check the value when the mic in silence or knocked sound. If the value doesn't change, something wrong with the mic input. You may need to tune the gain of mic module (sorry I don't know anything about the module you are using).

If the value changes corresponding to the sound loudness, you are very close to the goal.
Change the value "silent_thre" as the output value gets higher than it when you make a knock sound.

And setting the value "debug" to 1 will help you to see what happens.

Good luck,


mecha20122 years ago
Hye there. I tried to do this project for fun. I already got all the parts except the arduino coding. I already download it from the link attached but I can't open it with arduino software. This is error message when I tried to open it.

Exception in thread "AWT-EventQueue-0" java.lang.ArrayIndexOutOfBoundsException: 0

This is only part of the error message. Almost got it working except for the coding.


Is there any way I can get the coding working? Or at least I can open it?
longjie0723 (author)  mecha20122 years ago
Hmm, I tried the code with the newest arduino (1.0.4) on MacOS X, but no error to open. Which platform and version of the arduino are you using?
I have to restart my Arduino after about 5 cycles or knocks. Why is that? Otherwise, I have gotten it working with a piezo disc and no capacitor. Still I need to adjust the input values a little to get it working well. Love it!
Thank you!
longjie0723 (author)  stringstretcher2 years ago
I didn't notice that problem. I'll check it later. Thanks!
Great Job,
megaduty2 years ago
Good bathroom fun right there!
ashrat2 years ago
Good stuff!

Just wodnering, is the capacitor value of 100uF important? How would a higher value affect this?
longjie0723 (author)  ashrat2 years ago
Thank you! The capacitor value is not important. I'm not sure it is really needed. I encountered the problem the combination of sparkfun's MCM module and arduino board causes unstable output. I doubted the power quality from arduino, and added the capacitor. It seems help somehow.
Sugoi! Imoshiroi Arduino no hatsumei!

Anata no kodomo wa totemo kawaii! Nan sai desu ka?

Gomenasai, sukoshi nihongo wo hanasemasu.

(Instructable no website wa hiragana dekenai. Za nen.)
Nice! Interesting Arduino invention!
Your kid is really cute? How old is he?
I am sorry, something about letting something go
(the instructable website doesn't have hiragana. Too bad.)

Was I close? :P
I'm sorry, I only can speak a little Japanese. (I've forgotten so much since leaving.)

(Good job.)
longjie0723 (author)  JoshuaZimmerman2 years ago
Thank you, Arigato !

My son is 3 years old. He is enough brave to see a ghost ^^)
pbecker832 years ago
Havent seen an Arduino Vs yet. Cool. Here is a picaxe Vs a few years back.
longjie0723 (author)  pbecker832 years ago
Thank you! I haven't seen this project, it is brilliant! A lovely small box.
McMchriis2 years ago
Where I can GEt The Arduino Control!!!
Here is a list of distributors:
hasson2 years ago
Awesome, I'm definitely going to do this!
urosepsis2 years ago
Great job, I'm always on the look out for good pranks. My two girls will love it. Thank you.
DRH14692 years ago
That is halirous i'm lol lmao lmfao